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Amounts in Accumulated Other Comprehensive loss that have not yet been Recognized as Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2016
Sep. 30, 2015
Mar. 31, 2014
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]            
Actuarial loss (gain)   $ 2.5   $ 1.9 $ 8.6 $ 13.7
Pension Benefits [Member]            
Defined Benefit Plan Disclosure [Line Items]            
Actuarial loss (gain)       11.7 (54.1)  
Net actuarial loss due to curtailment         (0.6)  
Postretirement Benefits [Member]            
Defined Benefit Plan Disclosure [Line Items]            
Actuarial loss (gain) $ 0.9   $ 0.6 1.6 (1.3)  
Net actuarial loss due to curtailment     $ 3.5      
Other Accumulated Other Comprehensive Income | Pension Benefits [Member]            
Defined Benefit Plan Disclosure [Line Items]            
Beginning Balance 71.1     71.1 76.5  
Actuarial loss (gain)         (9.0)  
Current year actuarial gain       2.3 4.2  
Net actuarial loss due to curtailment         (0.6)  
Ending Balance       73.4 71.1 76.5
Beginning Balance 0.1     0.1 0.4  
Amortization         (0.1)  
Prior service cost recognition due to curtailment       (0.1) (0.2)  
Ending Balance         0.1 0.4
Total at December 31, 2016       73.4    
Other Accumulated Other Comprehensive Income | Postretirement Benefits [Member]            
Defined Benefit Plan Disclosure [Line Items]            
Beginning Balance 0.3     0.3 1.2  
Actuarial loss (gain)       (1.9) 0.4  
Current year actuarial gain       1.6 (1.3)  
Ending Balance         0.3 1.2
Beginning Balance $ (6.4)     (6.4) (21.2)  
Prior service cost recognition due to plan amendments       (12.2) 0.1  
Amortization       13.5 13.5  
Prior service credit recognition due to settlement         1.2  
Ending Balance       (5.1) $ (6.4) $ (21.2)
Total at December 31, 2016       $ (5.1)